Thin film transistors, known as TFTs, are widely used as switching elements in electronics, most notably for large area applications such as active matrix liquid crystal displays. The TFT is an example of a field effect transistor (FET). The best known FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high speed electronic applications. While the MOSFET specifically refers to SiO.sub.2 /bulk-Si transistors, more general combinations of Metal-Insulator-Semiconductors are known as MISFETs. The TFT is a MISFET in which the active semiconducting layer is deposited as a thin film.
Presently TFTs in most devices are made using amorphous silicon as the semiconductor. Amorphous silicon provides a cheaper alternative to crystalline silicon--a necessary condition for reducing the cost of transistors used in large area applications. Application of amorphous silicon is limited to slower speed devices since its mobility, .about.10.sup.-1 cm.sup.2 /V*sec, is 15,000.times. smaller than that of crystalline silicon. Although amorphous silicon is cheaper to deposit than crystalline silicon, deposition of amorphous silicon still requires costly processes, such as plasma enhanced chemical vapor deposition.
Recently organic semiconductors have received attention as potential semiconductor components for TFTs. See, for example, U.S. Pat. No. 5,347,144 to Garnier et al., entitled "Thin-Layer Field Effect Transistors With MIS Structure Whose Insulator and Semiconductor Are Made of Organic Materials". Organic materials (e.g., small molecules, short-chain oligomers, and polymers) may provide a less expensive alternative to inorganic materials for TFT structures, as they are simpler to process by methods such as spin-coating or dip-coating from solution, thermal evaporation, or screen printing. However, while the mobilities of organic materials have improved, their mobilities are still low and only the best materials have mobilities approaching that of amorphous silicon.
Organic semiconductors are less expensive and easier to deposit than conventional, amorphous silicon. Such organic materials are either small molecules (e.g. pentacene, metal-phthalocyanines), short-chain oligomers (e.g. n-thiophenes where n=3-8), or long-chain polymers (e.g. poly-alkylthiophenes or poly-phenylenevinylenes). Atomic orbital overlap between adjacent, multiply bonded atoms, known as conjugation, enables the transport of charge along molecules, oligomers, and polymers. Molecular orbital overlap between adjacent molecules enables inter-molecular charge transport.
Thin films of small molecules or short-chain oligomers show the highest mobilities for organic materials. The small molecules/short-chain oligomers showing these high mobilities have been deposited by thermal evaporation where they are deposited as highly ordered thin films. The high degree of ordering in the films provides orbital overlap and therefore charge transport between adjacent molecules. Long-chain polymers are advantageous since they are more soluble, enabling deposition by low cost techniques such as spin-coating and dip-coating, but have lower mobilities since they are more disordered.
While organic materials open up the possibility of depositing semiconductors for TFTs by cheaper and easier deposition techniques such as thermal evaporation, spin-coating, and dip-coating, their mobilities are still lower than desired. Typical mobilities for small molecules/short-chain oligomers range from 10.sup.-3 to 10.sup.-1 cm.sup.2 /V*sec and for long-chain polymers range from 10.sup.-8 to 10.sup.-2 cm .sup.2 /V*sec. The highest mobilities reported are 0.7 cm.sup.2 /V*sec for thin films of pentacene and 0.13 cm.sup.2 /V*sec for thin films of dihexyl,.alpha.-sexithiophene. A mobility of 0.3 cm.sup.2 /V*sec measured for single crystal .alpha.-sexithiophene represents the upper limit in mobility for this material. The mobilities of organic semiconductors rival those of amorphous silicon.
Organic-inorganic hybrid materials are a distinct class of materials which enable the combining of the useful characteristics of organic and inorganic components within a single material. Some members of this class of materials exhibit semiconducting characteristics. For the purposes of this description, an organic-inorganic hybrid material is a material composed of: organic components and inorganic components which are mixed together on a molecular level, and (i) wherein the material is characterized by a substantially fixed ratio of each organic component to each inorganic component; (ii) wherein at least one component is semiconducting; and (iii) wherein both organic and inorganic components manifest forces that enable a self assembly therebetween into a predictable arrangement.
One example of an organic-inorganic hybrid material takes the form of an organic-inorganic perovskite structure. Layered perovskites naturally form a quantum well structure in which a two dimensional semiconductor layer of corner sharing metal halide octahedra and an organic layer are alternately stacked.
For preparation of such organic-inorganic hybrid materials, spin coating techniques are suitable because many organic-inorganic perovskites are soluble in conventional aqueous or organic solvents. Using this method, high quality, highly oriented layered perovskite thin films have been achieved. Vacuum evaporation techniques have also been used to grow films of layered perovskites. Copending U.S. patent applications, Ser. No. 09/192,130, entitled "Single-Source Thermal Ablation Method for Depositing Organic-Inorganic Hybrid Films" U.S. Pat. No. 6,117,498; and Ser. No. 08/935,071, entitled "Two-Step Dipping Technique for the Preparation of Organic-Inorganic Perovskite Thin Films", U.S. Pat. No. 5,871,579, assigned to the same Assignee as this Application, both speak to alternative deposition methods for organic-inorganic hybrid materials. The disclosure of the aforementioned Applications is incorporated herein by reference.
Accordingly, it is an object of this invention to provide an improved FET structure which makes use of an organic-inorganic hybrid material as a semiconducting channel.
It is another object of this invention to provide an improved FET structure which may be processed at low cost.